1. Field of the Invention
The present invention relates to a thermal-mechanical process for the manufacture of precooked and dehulled corn flour and, more particularly, relates to a continuous column-roller process applied to the production of instant corn flour for use in the preparation of arepa and tortilla and derivatives thereof.
2. Description of Related Art
Corn represents one of the most important cereal grains for the nutrition of the Latin American population. The amounts consumed, as well as the form of consumption, are quite variable among different countries in the region. In Mexico and some Central American countries, the grain is transformed into food products by a unique process known as "nixtamalization", which basically consists of the alkaline cooking of whole corn kernel and breaking of the hull (or pericarp) down toward a partially gelatinized state (U.S. Pat. No. 4,513,018). Afterwards, the cooked corn is washed, ground and dried to give corn masa flour for different product applications. The main edible product of the traditional process is the "tortilla" which is a flat, round, unfermented and baked thin pancake produced from lime-cooked maize.
In the northern part of South America, particularly in Colombia and Venezuela, hard endosperm corn is processed with dry milling technology and it is further converted into a precooked flour for traditional maize foods. Its consumption is mainly in the form of "arepa" which is a flat, unleavened and baked thick pancake made from dehulled and degermed cooked-maize. In other South American countries, corn meal and corn flour are used for different bakery and pancake mixes as well as snack products.
At the present time, there is a large expansion in the consumption of corn products in the U.S.A. and tortilla is one raw material for the preparation of snack and traditional "Tex-Mex" dishes. Thus, there is an increasing demand and use of industrially prepared corn flours so they can provided benefits to the consumer, such as convenience, availability, price and a more stable quality. The potential market for masa flours in the U.S.A., Mexico and Central America is estimated at 17.8 million tons per year based on an average annual per capita consumption of 122 kilogram (tortilla and snacks: Sustain, 1997).
In the production of corn products, such as tortillas, chips and the like, from food grade corn, it is known that the hard endosperm corn (U.S. Pat. No.2: USFGC, 1996) must be partially cooked before it is formed into the end product, so as to cause it to be a partially gelatinized corn flour. In the past, this has been done by processes wherein the corn is cooked and/or steeped in a lime-water solution (traditional nixtamalization) such as those disclosed in U.S. Pat. Nos. 2,584,893 (Lloyd et al.), 3,194,664 (Eytinge) and 4,326,455 and 4,513,018 (Rubio), and subsequently ground and dried to produce nixtamalized corn flour (masa flour) from which such corn products may be made. As it may be seen from the above, the prior art methods for the industrial production of corn dough or masa flour involve accelerated cooking times with large amounts of processing water and increased costs for water removal. The lime-cooking changes produced in the hull or pericarp fraction are connected with the hydrolysis and solubilization which releases a highly hydrated dietary fiber having a marked effect on the rheological and mechanical characteristics of the traditional dough and its tortilla. Furthermore, the lime-water residue ("nejayote") is composed of 2.2 to 2.8% total solids from a continuous cooker which included 64-76% dietary fiber, 12-20% starch and 1.4% protein. The raw corn quality, cooking-time profile and process-type (Alvarez and Ramirez, 1995) utilized have been proposed as the main variables which determine corn solid losses in nejayote.
With the aim of solving the above described problems by the traditional and/or industrial processes, some workers have developed alkaline-cooking methods with extruders. In this connection reference is made to the following U.S. Pat. Nos: Rubio 4,250,802, Martinez-Bustos et al. 5,532,013 and Bazua et al. (1979). Although the above-described prior art provides means for rendering grain, such as corn or dehulled corn, into corn dough and/or masa flour under process conditions involving reduced amounts of water with shortened processing times, an industrial extruder capable of producing such a corn dough or masa flour was still unavailable in the market.
In view of the limitations of the prior art accelerated methods for alkaline-cooking (thermal process) and alkaline-extrusion (thermal-mechanical process) for producing instant masa flour, they are not yet suitable for obtaining a corn flour with "different degrees of starch gelatinization" during precooking of the corn endosperm. In accelerated alkaline-cooking with simultaneous "impact grinding and flash drying processes", the peripheral zone of the endosperm might present starch gelatinization and, while this permits an increase in water adsorption (dough yield) during preparation of the dough, it may also facilitate water evaporation (dehydration) during tortilla cooking, whereby the mechanical properties are negatively affected by hardening or staling of the tortilla without texture additive.
In view of the above, Wimmer et al. in U.S. Pat. No. 3,404,986 proposed a mechanical method for separating germ, endosperm and hull (pericarp) of the corn kernel and then prepared mixtures of moisturized standard corn meal and degerminated meal or flour (containing 18 to 35% moisture) and subjected it to the action of heated rotating rolls to cause controlled starch gelatinization (precooking) with subsequent drying and grinding to produce a corn flour. This instant corn flour can be used for a reduced fat snack and may be used in tortilla preparation. Hart, in U.S. Pat. No. 4,329,371, describes a hybrid method without the use of alkali-cooking, by providing a step of dehulling the corn grains with a dehusking apparatus, separating the hull from the endosperm and germ fractions, using the hull for other low-value applications, and cooking the endosperm and germ fractions with steam. This cooked-dehulled fraction is then dried and ground to produce a corn flour for tortilla.
Vaqueiro et al., in U.S. Pat. No. 4,594,260, discloses another hybrid method of obtaining corn flour which essentially comprises removing the hull from the germ and endosperm of the corn kernel until a hull-containing fraction and an endosperm-germ-containing fraction are produced and then nixtamalizing by the traditional method only the hull fraction and remixing the same with the uncooked endosperm-germ fraction (containing 30 to 35% moisture). This moisturized mixture is ground and dried to obtain an instant corn masa flour used in producing tortilla. Herbster, in U.S. Pat. No. 5,176,931, defines a short-time, low-pollution hybrid method of preparing masa flour for making tortilla chips. This procedure preferably comprises subjecting alkali-treated debranned and moisturized corn (15 to 35%) to near infrared radiation in order to partially cook the dehulled grain. The precooked-dehulled fraction can be additionally cooked, cooled and milled to desired particle size similar to industrial masa flour (Masa mixta brand) having 55% of non-gelantinized starch granules, 25% partially gelatinized and 20% fully gelatinized (according to microscopic examination).
According to Watson (1987), the corn hull or pericarp (bran) makes up 5-6% of the kernel dry weight. In reference to the value-enhanced corn report (USFGC, 1996), a white-hard-endosperm kernel may contain: 11.5-11% moisture, 72.2-73.2% starch, 10.5-9.8% protein, 4-3.7% fat, and 2.1-2.3% of ash and crude fiber. Furthermore a dry-milled sample might yield, on a dry weight basis, 76.2-74.8% of total endosperm, 18.9-20.5% of germ and 4.9-4.8% of pericarp. A typical materials balance for a Venezuelan arepa plant can yield a minimum of 65% flour on a wet basis (Cuevas, 1985). By 1993, a whole or integral arepa flour was produced with more fiber and less protein and fat as compared to the traditional degermed flour which is enriched by Venezuelan law with vitamins (B and A) and iron.
Fiber Components of Corn Kernel Parts % Dry Fiber Fiber Fiber % Kernel Part Matter Insol. Hemicellulose Cellulose Lignin Soluble Total Fiber Whole Kernel 100 9.5 6.7 3 0.2 0.1 9.5 100 Starch endosperm 80.9 1.0 -- -- -- 0.5 1.5 12 Aleurone endosperm 2.0 50.0 -- -- -- 25.0 75.0 15 Germ 11.0 11.0 18 7 1.0 3.0 14.0 16 Pericarp (bran) 5.3 90.0 67 23 0.1 0.6 90.7 51 Tip cap 0.8 95.0 70 -- 2.0 -- 95.0 6